NGC 7217, A Multi-Ring Galaxy

Optics:	Ritchey–Chrétien 20" F/8.2 (4166mm FL)	Processing:	PixInsight, Photoshop
Camera:	SBIG STXL-11000 with Adaptive Optics	Date:	October 2022
	11 Megapixel (4008 x 2672 16-bit sensor)	Location:	Columbus, Texas
Exposure:	LRGB = 640:120:80:100 minutes	Imager:	Kent E. Biggs

NGC 7217 is truly a fascinating spiral and ringed galaxy located in the direction of the constellation Pegasus, the winged horse. The galaxy has multiple rings of billions of stars in formation about its central nucleus* which consists of a supermassive black hole surrounded by a billion or so stars itself. Starting from just outside the galaxy traveling inward, we encounter ghostly spheroidal halo surrounding it; the halo contains countless stars and is so distant that it does not resolve into individual points of light. The halo appears more circular than the galaxy since it is evenly distributed in all directions, whereas the galaxy’s rings are at an angle to us, appearing elliptical.

Next, we encounter a near perfect massive ring of younger stars glowing blue with bands of darker material of dust and fainter stars labelled Galaxy Ring*. A gap then appears reminiscent of the normal star distribution in other spiral galaxies. Inside that gap we see a clear Inner Ring* of young hot stars once again followed by another yellowish-brown gap. In the image below you can see even another clear gap inside that which seems to be a near perfect circle around the nucleus. With the help of Hubble, astronomers have discovered several other rings inside this inner visible one. Additionally, that central region experienced several starbursts in which an exceptionally high rate of star formation occurs.

Another fascinating attribute of NGC 7217 comes from astronomers studying its rotation. Data revealed that 20-30% of stars are rotating around the galaxy’s core in the opposite direction as other stars. This almost certainly indicates a merger with another galaxy, although NGC 7217 seems currently isolated in space at the time light left the galaxy about 50 million years ago! Whether that merger took place early on in the galaxy’s formation, or more recently is still a mystery.

Visible in the image above are dozens of other faint galaxies; two have inset enlargements*. They are in fact much more distant and do not even have a PGC designation which means they are not included in the PGC million galaxy catalog.

Another interesting observation is concerning NGC 7217’s spherical halo. While all stars in images from earth are in our own galaxy, they will therefore visually appear in front of any other galaxy. Yet what you think are thousands of faint smudgy background stars in this image are not visible with the same density in front of the halo. This absence tells us that most of those distant smudgy stars are actually distant faint galaxies and therefore hidden behind the halo!

*See annotations by hovering the pointer over the above image.

NGC 7217 with and without Stars

NGC 7217 Zoomed-In with and without Stars

NGC 7217, Enlarged with Gen2 and Gen3 processing compared

Gen1 and Gen2 Processing compared.

Finally, the the above image compares the latest image processing techniques using PixInsight, the RCOS 20 inch telescope, and the 10 megapixel SBIG STXL-11000 camera with the original image process techniques using MaxImDL, the 1st generation 3 megapixel SBIG ST10-XME camera, and the Celestron 11 inch telescope. Hover over the image to see the before and after comparison.